In both the administration of transfused blood as well as the recirculation of blood from the cardiotomy reservoir during open heart surgery, there may be considerable debris which is removed before the blood is returned to the patient. The blood from the cardiotomy reservoir may contain platelet aggregates, leucocyte aggregates and lipid aggregates or some combination of these which should be removed before the blood is returned to the patient. Transfused blood depending on how long it has been stored may contain the above type of aggregates as well as fibrin clots, agglomeration of protein precipitates and other undesirable debris.
Presently various blood filters are offered which have the objective of removing this undesirable debris from the blood. The media used in these filters are either a surface type filter media or a depth type filter media. The surface media are generally a woven fabric having pore sizes ranging from 20 microns to 170 microns or larger and actually act as a sieve in the filtration. The depth media are either loose fiber packs or foam structures which have a depth of from a few thousandths of an inch to an inch or more, and which have a range of pore size.
Some of the debris in the blood being filtered is deformable. Also some of the blood forms aggregates, which when they contact the media may break up, pass through the media and immediately reagglomerate unless removed by the filtering media. Because of these type of debris, the sieve filters do not do a sufficient job in filtering either transfused blood which has been stored for any length of time or in filtering blood from the cardiotomy reservoir in open heart surgery. Sieve filters or surface media will allow debris that is considerably larger in micron size than the pore opening to be detected in the blood after passing through the filter.
The depth media which filter on a theory of the debris adhering to the surface do not suffer from the same problems as the sieve type units. However, the depth media, such as loose fiber packs, will tend to channel after considerable use such as a number of hours in an extracorporeal circuit used in open heart surgery or after a number of units of whole blood have been filtered as happens in some transfusions. During use, the pressure drop across the media will increase and will create channels in the media and increase the effective pore size of the media allowing some debris to pass through and increases the possibility of reagglomeration as previously described. The flexible open cell foams, though they don't channel, will tend to change pore dimensions under different pressure drops. Hence, as the foam filter becomes blocked and the pressure increases some pores may be closed while adjacent pores are opened or made larger and the effective pore size of the foam is altered considerably.